I want to start short day onion seeds in trays, and I have T5 grow lights . But I'll have to plant them outside in the ground after a month or two and if I start now that'll be in the middle of winter. I'm in the Los Angeles area but up in a canyon where it's a bit colder at night. Will onions keep growing in an area with plenty of nights in the 30's with the very occasional brief drop to 26-28F? ' Or should I wait until January to start them indoors in trays. Any suggestions?

I buy bare-root onions every year (9 cents each at the local nursery) and plant them now. Iirc I’ve planted as late as December. Then I harvest in May, or earlier if the gophers have started harvesting them.
So if you’re starting from seed, now is good. I’d be more concerned about the temperatures in late spring. I got alarmed a couple years ago when they all started sending up flowering stalks too early, but i stopped watering and got a good harvest.

Onions do fine in cold weather.
You could probably direct sow right now and think later for green onions.
Southern California does not get cold enough to bother onions.
What you can have a problem with is temperature fluctuations making them think they went through 2 seasons and they'll bolt in the spring before they start to bulb up.
Onions won't bolt until they get about 6 or 7 leaves.

Considering where you are you have a little over 14 hours daylight.
You could get away with mid day length onions.
That would give the onions more time to make more leaves before the bulbed up.